Use tree-diffing for difference

[sjakobi]

Context: #364

[sjakobi]

Wow, the -fdebug assertions are paying off, I guess!

         valid:                             FAIL
          *** Failed! (after 54 tests and 15 shrinks):
          Exception:
            Data.HashMap.Internal.Array.shrink: Check failed:  _n >  (0 :: Int) (0 vs. 0)
            CallStack (from HasCallStack):
              error, called at ./Data/HashMap/Internal/Array.hs:225:39 in unordered-containers-0.2.20.1-inplace:Data.HashMap.Internal.Array
          fromList [(K {hash = -1, _x = B},0),(K {hash = -1, _x = A},0)]
          fromList [(K {hash = -1, _x = A},0),(K {hash = -1, _x = B},0)]
          Exception thrown while showing test case:
            Data.HashMap.Internal.Array.shrink: Check failed:  _n >  (0 :: Int) (0 vs. 0)
            CallStack (from HasCallStack):
              error, called at ./Data/HashMap/Internal/Array.hs:225:39 in unordered-containers-0.2.20.1-inplace:Data.HashMap.Internal.Array
          
          Use --quickcheck-replay="(SMGen 2214100628715094315 10962607274683585473,53)" to reproduce.
          Use -p '/Data.HashMap.Strict.difference.valid/' to rerun this test only.

[sjakobi]

So the failing assertion is this one, introduced in b73381e:

unordered-containers/Data/HashMap/Internal/Array.hs

Lines 221 to 228 in 59ddae5

	-- | The returned array is the same as the array given, as it is shrunk in place.
	shrink :: MArray s a -> Int -> ST s (MArray s a)
	shrink mary _n@(I# n#) =
	CHECK_GT("shrink", _n, (0 :: Int))
	CHECK_LE("shrink", _n, (unsafeLengthM mary))
	ST $ \s -> case Exts.shrinkSmallMutableArray# (unMArray mary) n# s of
	s' -> (# s', mary #)
	{-# INLINE shrink #-}

I don't see an obvious technical reason why an array shouldn't be shrunk to length 0, apart from the fact that an array of length 0 is useless for storing elements. In the case of the new filter function, an array of length 0 seems like a perfectly valid result.

So I'll just change CHECK_GT to CHECK_GE.

[treeowl]

I agree that assertion is bogus.